An organic EL (electroluminescence) element has a structure in which an organic functional layer including a light-emitting layer is sandwiched by an anode and a cathode. In this structure, when a voltage is applied, holes and electrons injected from the anode and cathode into the light-emitting layer recombine to self-emit light.
An organic EL element usually has low resistance to the oxygen and moisture in the air. Typically, the organic EL element will degrade, whereby a phenomenon occurs in which non-light-emitting portions known as “dark spots” occur and expand in the light-emitting region. Although there are various opinions regarding the mechanism in which dark spots occur, it is believed that one factor is the inhibition of the injection of electrons from the cathode into the adjacent organic layer due to oxygen, moisture and the like that have infiltrated into the element.
As the cathode material, to obtain good electron injection, a material having a low work function is used, such as Mg:Ag, Li:Al, Ca, and Li2O/Al. Consequently, the cathode is easily oxidized by oxygen and moisture in the air. For example, if Al is used, AlOx is formed. Thus, depending on the material an insulating oxide film is formed. Dark spots occur due to reasons such as the cathode at a site that has been oxidized and degraded loses its function of injecting electrons, and localized increases in interfacial resistance.
Conventionally, one measure taken to suppress oxidation of the cathode has been to use a film sealing structure, in which the element is sealed by forming a thin film of a material having low permeability to oxygen and moisture, such as silicon nitride (SiNx). However, even if such a sealing structure is used, it is difficult to completely prevent the occurrence and expansion of dark spots due to the infiltration of oxygen and moisture from defective portions, such as pin holes and cracks. In particular, organic EL elements, which have the advantage that they can be formed on a flexible substrate such as plastic, suffer from the problem that cracks tend to form when the element is flexed.
Among conventional organic EL elements, a structure having a hole-electron current conversion layer in which an electron transport layer and a hole transport layer are laminated with a heat-reducible metal layer interposed therebetween, is known (for example, see Patent Literature 1). This hole-electron current conversion layer is provided to efficiently convert a hole current into an electron current. However, a heat-reducible metal such as Al is also easily oxidized like the cathode, so that there is the problem that if an oxide film is formed, the hole-electron current conversion efficiency deteriorates. In addition, as can be seen from the results in FIGS. 8 and 9 of Patent Literature 1, light is not emitted unless the heat-reducible metal is present. Thus, for the organic EL element structure described in Patent Literature 1, a heat-reducible metal is an essential component. Therefore, aside from the cathode problem, the occurrence of dark spots due to oxidation of the heat-reducible metal is another problem.
Further, in the organic EL element of Patent Literature 1, if the cathode formed from a material having a low work function such as Li is oxidized, there is also the problem that the charge transfer complex that is necessary for the element to emit light is not formed.